Well, the Mistresses have weighed in! Seriously though, without knowing what you can afford, I would recommend going to two hours before I would recommend two Mistresses if this is completely new to you. One hour feels like you were just getting started and makes it difficult to ease in slowly, especially if you want to try different things!
Then again, if your focus is on tickling and you don't have a lot of other activities you want to try, feeling outnumbered and bullied could be a lot of fun for a short session. You could have a tickling contest where each Mistress gets a minute to see who can make you laugh hardest before they all join in together! But always leave yourself something to expand to in future sessions!